1. Field of the Invention
The present invention relates to a light emitting device using as a light source an element (hereinafter referred to as “an organic EL element”) which is composed of an anode layer, a cathode layer, and a layer including an organic compound (hereinafter referred to as “an organic compound layer”) in which an electroluminescence (hereinafter referred to as “an EL”) is generated by applying an electric field thereto. The EL in the organic compound includes luminescence generated when it is returned from a singlet excitation state to a ground state (fluorescence) and luminescence generated when it is returned from a triplet excitation state to a ground state (phosphorescence). In particular, the present invention relates to a light emitting device in which a porous body of an inorganic material is made in contact with an organic compound layer and thus the generation of phosphorescence is promoted. Note that a light emitting device in this specification indicates an image display device or a light emitting device using the organic EL element as a light emitting element. Also, a module in which a TAB (tape automated bonding) tape or a TCP (tape carrier package) is attached to the organic EL element, a module in which a printed wiring board is provided in the end of the TAB tape or the TCP, and a module in which an IC (integrated circuit) is directly mounted on the organic EL element by a COG (chip on glass) method are included in the light emitting device.
2. Description of the Related Art
An organic EL element is an element for emitting light by applying an electric field thereto and noted as a next generation flat panel display element because of characteristics such as lightweight, direct current low voltage drive, and fast response. Also, the organic EL element is a self light emission type and has a wide view angle. Thus, it is considered that the organic EL element is effective as the display screen of a mobile equipment.
It is said that a light emitting mechanism of the organic EL element is said that an electron injected from a cathode and a hole injected from an anode are recombined to form a molecule with an excitation state (hereinafter referred to as “a molecular exciton”) and the molecular exciton releases energy to emit light when it is returned to a ground state. As the excitation, a singlet state (S*) and a triplet state (T*) are allowed, and it is considered that its statistical generation ratio is S*:T*=1:3 (Reference 1: Junji Kido, “Monthly Display Separate Volume, Organic EL Display, from Fundamentals to current information” (TechnoTimes Co., Ltd.), pp. 28–29).
However, with respect to a general organic compound, light emission (phosphorescence) with the triplet excitation state (T*) is not observed at a room temperature. This is caused even in the case of the organic EL element and only light emission (fluorescence) with the singlet excitation state (S*) is generally observed. Thus, it is assumed that a theoretical limitation of internal quantum efficiency (ratio of the photon generated to the injected carrier) in the organic EL element is 25% based on evidence with S*:T*=1:3.
Also, all generated lights are not emitted to the outside and a portion of the lights cannot be picked up because of the refractive indexes inherent to organic EL element constituent substances (organic compound layer and electrodes) and a substrate. A ratio of the light picked up in the outside to the generated light is called light pickup efficiency. It is said that the pickup efficiency in an organic EL element having a glass substrate is about 20%.
From the above reason, even if all the injected carriers form the molecular excitons, it is said that the theoretical limitation of a ratio of photons (hereinafter referred to as “an external quantum efficiency”) finally picked up in the outside of the organic EL element to the number of injected carriers is 25%×20%=5%. That is, even if all the carriers are recombined, only 5% of the recombined carriers are picked up as light according to calculation.
However, recently, organic EL elements capable of converting energy released when it is returned from a triplet excitation state to a ground state (hereinafter referred to as “triplet excitation energy”) into light to be emitted are successively reported and these high light emission efficiencies are noted (Reference 2: D. F. O'Brien, M. A. Baldo, M. E. Thompson, and S. R. Forrest, “Improved energy transfer in electrophosphorescent devices”, Applied Physics Letters, Vol. 74, No. 3, 442–444 (1999) and Reference 3: Tetsuo Tsutsui, Moon-Jae Yang, Masayuki Yahiro, Kenji Nakamura, Teruichi Watanabe, Taishi Tsuji, Yoshinori Fukuda, Takeo Wakimoto, and Satoshi Miyaguchi, “High Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center”, Japanese Journal of Applied Physics, Vol. 38, L1502–1504 (1999)).
In Reference 2, a metal complex with platinum as main metal (hereinafter referred to as “a platinum complex”) is used. Also, in Reference 3, a metal complex with iridium as a main metal (hereinafter referred to as “an iridium complex”) is used. Thus, it can be said that a characteristic is to introduce a third transition series element as a main metal in any metal complex. Of those, there is a metal complex in which the theoretical limitation value of the external quantum efficiency as described above greatly exceeds 5%.
As described in References 2 and 3, with respect to the organic EL element capable of converting the triplet excitation energy into light to be emitted, higher external quantum efficiency than a conventional element can be achieved. In addition, if the external quantum efficiency is increased, a light emission intensity is improved. Thus, it is considered that the organic EL element capable of converting the triplet excitation energy into light to be emitted constitutes a large share toward future developments as a manner for achieving high intensity light emission and high light emission efficiency.
However, since both platinum and iridium are so-called noble metal, the platinum complex and the iridium complex using these metals are expensive and thus it is expected that a cost reduction is hindered in future. In addition, when the influence of a metal complex including heavy metal on the human body is considered, a material, which is safe and for which waste processing is easily performed is desirable.
Therefore, it is desirable that the organic EL element capable of converting the triplet excitation energy into light to be emitted (that is, light is emitted as phosphorescence), without using the iridium complex and the platinum complex, which are already existed, is developed. A most simple method is for developing a new organic compound in which light is emitted as phosphorescence at a room temperature with a low cost. However, a clear molecular design plan is not established until now and it is very difficult in many sides.
Thus, although it is important to develop new phosphorescence light emission material, it is the situation that a method of designing an element structure such that phosphorescence light emission is promoted with respect to an organic EL material is desirable.
Also, with respect to the organic EL elements capable of converting the triplet excitation energy into light to be emitted, as described in References 2 and 3, there is a problem in an element life, that is, the half life of intensity does not reach a practicable level. According to the report example of Reference 3, when an initial intensity is set to be 500 cd/m2, the half time of the intensity is about 170 hours.
Even if the high intensity light emission and the high light emission efficiency are achieved, the element life is very important for practicable use. Thus, it is said that an element structure effective to stabilization of an element is desirable if possible.